1. Field of the Invention
The invention relates to a hydrometallurgical field, and more particularly to a method for extracting tungsten from scheelite.
2. Description of the Related Art
A typical method for processing scheelite includes an autoclaving step in which sodium carbonate is used as a leaching agent. The method can stably decompose scheelite, yielding a product in which residual WO3 can be controlled to be less than 1%; however, the reagent dosage requirements are too large to render this method practical: about three times of the theoretical dosage. In some lab experiments, the dosage required to practice this method can reach even 5-6 times of the theoretical dosage. Furthermore, the operation temperature for this method is very high, about 225° C., and the pressure of the device is 20 atm.
Another method for processing scheelite includes a step of autoclaving using NaOH as a leaching agent, and more than 80% of ammonium paratungstate (APT) produced in China occurs by this method. The method is carried out by adding large amounts of NaOH in a high concentration and at a high temperature and pressure, so that the scheelite can be effectively decomposed. Residual WO3 in the product can be lowered to 1-3%. However, the technique consumes large amounts of energy, the production costs are high, and large amounts of wastewater are produced in the subsequent process.
Acid decomposition is a method that mainly uses hydrochloric acid to process scheelite concentrate. A thermodynamic study of this method proved that the reaction rate is fast. However, during the hydrochloric acid decomposition, tungstic acid in the form of a yellow gel can wrap around undecomposed scheelite particles, which results in incomplete decomposition. The hydrochloric acid generates serious problems for the processing step by generating acid corrosion and volatilization of the acid. These effects produce poor working conditions. Finally, after treating the remnant mother liquor of the hydrochloric acid with lime, large amounts of a CaCl2 solution are discharged. For these reasons, this method has been discarded.
Tungsten, phosphorus, arsenic, silicon, and other impurities can form a soluble heteropoly acid (for example, [PW12O40]3−) having a 1:6-1:12 ratio of impurities to tungsten. During the hydrochloric acid decomposition, even small amounts of phosphorus can cause the loss of large amounts of tungsten into the leaching solution. The hydrochloric acid decomposition step is mainly used to process scheelite concentrates that are highly pure, with very low content levels of phosphorus, arsenic and other impurities. However, it implies us that the intentional addition of a small amount of phosphoric acid during the leaching process can cause tungsten in the solution to form a soluble phosphotungstic heteropoly acid. The problem associated with tungstic acid wrapping during the hydrochloric acid decomposition can be overcome. Studies have shown that the yellow tungstic acid still forms at low levels of phosphorus; therefore, a large excess coefficient for phosphorus is required. Large quantities of phosphorus increase the leaching speed. Corrosion and volatilization problems associated with the hydrochloric acid still exist; thus, the results of the study have not been applied in industry.
To address the problem associated with the corrosion and volatilization of hydrochloric acid and to realize the tungsten leaching in the form of a soluble phosphotungstic heteropoly acid, sulfuric acid can substitute for hydrochloric acid. However, in the presence of a large amount of sulfuric acid, the supersaturated gypsum quickly nucleates to form a large amount of fine crystals that produce wrapping. The decomposition effects are, therefore, insufficient. During the sulfuric acid decomposition step, phosphoric acid, calcium phosphate, or phosphorite is added to provide phosphorus as a complexing agent for tungsten. A certain amount of NaCl is also needed to improve the decomposition. Then, HCl in the strong sulfuric acid-containing solution has a high degree of activity due to the high concentration of hydrochloride acid. Thus, problems like the corrosion of Cl− come out again.
The principles underlying the action of NaCl are as follows: the chloride ions can significantly increase the induction period of the calcium sulfate crystals, thereby preventing the calcium sulfate crystals from nucleating. To a certain degree, it is helpful to form large crystals and prevent the products from wrapping the minerals and obstructing the decomposition reaction. However, in actual fact, adding NaCl cannot promote ideal decomposition; rather, it raises problems such as device corrosion and HCl volatilization. NaCl cannot prevent tungstic acid wrapping.